This invention relates to surface crosslinked and surfactant coated water-absorbent resin particles. It further relates to an improved method for preparing surface crosslinked and surfactant coated water absorbent resin particles. In another embodiment, the invention relates to absorbent structures incorporating the absorbent resin particles of this invention.
Water absorbent resin particles (also referred to as superabsorbent polymers) are primarily used in personal care products to absorb body fluids, for example baby diapers, adult incontinence products, feminine hygiene products, and the like. In this application, the absorbent resin particles are incorporated into absorbent structures, for example synthetic and natural fiber or paper based woven and nonwoven structures, and toughened masses of fibers, such as fluff pads. The materials used in such structures can instantaneously absorb aqueous fluids and distribute them over the whole absorbent structure, in the case of fiber based materials by capillary forces. The structures, in the absence of water-absorbent resin particles, have limited absorption capacity, and are very bulky due to the large amount of material needed to provide acceptable absorption capacity. Further, the absorbent structures do not retain fluid under pressure. A means for improving the absorbency characteristics is to incorporate in the absorbent structures water absorbent resin particles which imbibe fluid to form a swollen hydrogel material, see U.S. Pat. No. 4,610,678 (incorporated herein by reference). This hydrogel serves to retain the absorbed fluid even under pressure and gives the absorbent structure a "dry feel" even when wetted. In order to reduce the bulk of absorbent structures containing water absorbent resin particles, large volumes of the absorbent structure material can be replaced with small volumes of absorbent resin particles. The absorbent resin particles must quickly absorb fluids and retain such fluids to prevent leakage.
Water-absorbing resins known in the art include carboxyl-containing resins selected from the group consisting of a hydrolyzate of a starch-acrylonitrile graft polymer, a partially neutralized product of a starch-acrylic acid graft polymer, a saponification product of a vinyl acetate-acrylic ester copolymer, a hydrolyzate of an acrylonitrile copolymer, a crosslinked product of a hydrolyzate of an acrylamide copolymer, a hydrolyzate of an acrylamide copolymer, a crosslinked product of a hydrolyzate of an acrylamide copolymer, a partially neutralized product of polyacrylic acid, and a crosslinked product of partially neutralized polyacrylic acid. These water-absorbing resins are well known in the art. For example, the hydrolyzate of a starch-acrylonitrile graft polymer is disclosed in U.S. Pat. No. 3,661,815 (incorporated herein by reference); the neutralization product of a starch-acrylonitrile acid graft polymer is disclosed in U.S. Pat. No. 4,076,663 (incorporated herein by reference); the saponification product of a vinyl acetate-acrylic ester copolymer is disclosed in Japanese Laid-Open Patent Publication No. 14689/1977 (incorporated herein by reference); the hydrolyzate of an acrylonitrile copolymer and the hydrolyzate of an acrylamide copolymer are disclosed in Japanese Patent Publication 15959/1978 (incorporated herein by reference); the crosslinked products of these hydrolyzates and a self-curable poly(sodium acrylate) obtained by inverse phase suspension polymerization are disclosed in U.S. Pat. No. 4,093,776 (incorporated herein by reference); and the crosslinked product of partially neutralized polyacrylic acid is disclosed in Japanese Laid-Open Patent Publication 84304/1980 (incorporated herein by reference).
Japanese Laid-Open Publication 84632/1981 (incorporated herein by reference) discloses an absorption agent consisting of a crosslinked poly(alkali metal acrylate) material obtained from (A) 0.01-10 parts by weight of a water-soluble and/or water dispersible surfactant (B) 0.005-20 parts by weight of a water soluble polyvalent alcohol per 100 parts by weight of (C) an acrylate salt based polymer composed from 10-40% by mole of acrylic acid and 60-90% by mole of acrylic acid alkali metal salt where the said acrylate salt based polymer (C) is solution polymerized in an aqueous solution at a concentration of at least 30% by weight to form a gel like water containing polymer which is subsequently heated and dried.
In conventional water-absorbent resins increasing the water-absorption capacity results in an increase in the fraction of water-soluble polymer present. At the same time the gel strength of the swollen gel, its absorption capacity under pressure and absorption speed is reduced. The water absorbent resin tends to agglomerate upon wetting, and such agglomeration results in a reduction of its absorption capacity. In absorbent structures containing such water absorbent resins, when agglomeration occurs after wetting gel blockage can result which prevents transport of fluids within the absorbent structure. This is a particular problem when a large fraction of the absorbent structure is replaced with water absorbent resin particles to prepare a thin absorbent device.
Several approaches are known for improving the absorption characteristics of aqueous fluid absorbent resins. Several focus on polymer post treatment, more specifically polymer surface modification, i.e. post surface crosslinking. EP 0,248,963 (incorporated herein by reference) describes a process of polymer post treatment by which the surface of a water-absorbent resin is treated with polyquarternary amines to increase the absorption rate significantly and the adsorption under load (AUL) by 10%. In this process the polyquarternary amines are applied as solutions in methanol. A separate blending operation is required to evenly distribute the polyquaternary amine evenly throughout the resin.
EP 0,248,437 (incorporated herein by reference) describes a process for polymer surface post crosslinking in which an aqueous solution of a water soluble peroxide radical initiator is sprayed onto the surface of the water absorbent resin particles and the coated particles are heated. Additional surface crosslinking is claimed leading to a product of improved water absorbency and water absorption rate. It is disclosed that the uniformity of penetration of the aqueous solution into the surface of the absorbent polymer may be improved in this process by use of a water soluble organic solvent such as methanol. A problem with this process is that high levels of peroxide radical initiators discolor partly neutralized polyacrylic acid and increase the low molecular weight water soluble fraction of this product if it is subjected to heat.
A process is disclosed in DE 3,713,601 A1 (incorporated herein by reference), in which surface crosslinking is obtained by the addition of a crosslinker of glycidyl- or polyglycidyl compounds. These crosslinking agents irritate the human skin, and may be harmful to human health.
A polymer post-treatment process is disclosed in GB 2,119,384, (DE 3,523,617 A1) (incorporated herein by reference) in which improvement of product properties is claimed by mixing absorbent resin powder with crosslinkers, preferably polyalcohols, like glycerol, a solvent and water, and heating this mixture to temperatures in the range of 90.degree.-300.degree. C. It is claimed that this post treatment reduces the agglomeration of the absorbent resin when wetted, increases its speed of water absorption and water retention, improves the flowability and reduces the dust. This process has some disadvantages. Traces of water are quickly absorbed by superabsorbent polymers, rendering it difficult to mix additives which are applied as an aqueous solution, with the dry, powdered superabsorbent. Addition of an aqueous solution to the polymer powder results in particle agglomeration, and when the agglomerated particles are subjected to heat, they form hard clusters which make additional processing such as breaking and screening necessary. It is disclosed in the reference that if the polyalcohols are added without a solvent, good distribution is not possible. The polymer coating formulation therefore preferably contains a hydrophilic organic solvent, e.g. methanol. The use of an organic solvent is not desirable due to its processability and its negative impact to the environment.
Great Britain Patent 2,162,525 (incorporated herein by reference) discloses the addition of finely divided silica to the crosslinked adsorbent resin particles as described in Great Britain Patent 2,119,384, to prevent agglomeration of the resin particles.
What are needed are water absorbent resin particles which have high absorption capacity, low extractables, i.e. water-soluble polymer fraction, and high toughness or gel modulus. Furthermore, what are needed are water absorbent resin particles which have a low tendency to agglomerate. What is further needed is a process for the preparation of such particles, which is environmentally friendly. What are further needed are particles and processes which do not employ the use of materials which may be irritating, or which may be harmful to human health.